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Legg J.P.,International Institute Of Tropical Agriculture | Sseruwagi P.,Mikocheni Agricultural Research Institute | Boniface S.,International Institute Of Tropical Agriculture | Okao-Okuja G.,National Agricultural Crops Resources Research Institute | And 10 more authors.
Virus Research | Year: 2014

The greatest current threat to cassava in sub-Saharan Africa, is the continued expansion of plant virus pandemics being driven by super-abundant populations of the whitefly vector, Bemisia tabaci. To track the association of putatively genetically distinct populations of B. tabaci with pandemics of cassava mosaic disease (CMD) and cassava brown streak disease (CBSD), a comprehensive region-wide analysis examined the phylogenetic relationships and population genetics of 642 B. tabaci adults sampled from cassava in six countries of East and Central Africa, between 1997 and 2010, using a mitochondrial DNA cytochrome oxidase I marker (780 bases). Eight phylogenetically distinct groups were identified, including one, designated herein as 'East Africa 1' (EA1), not previously described. The three most frequently occurring groups comprised >95% of all samples. Among these, the Sub-Saharan Africa 2 (SSA2) group diverged by c. 8% from two SSA1 sub-groups (SSA1-SG1 and SSA1-SG2), which themselves were 1.9% divergent. During the 14-year study period, the group associated with the CMD pandemic expansion shifted from SSA2 to SSA1-SG1. Population genetics analyses of SSA1, using Tajima's D, Fu's Fs and Rojas' R2 statistics confirmed a temporal transition in SSA1 populations from neutrally evolving at the outset, to rapidly expanding from 2000 to 2003, then back to populations more at equilibrium after 2004. Based on available evidence, hybrid introgression appears to be the most parsimonious explanation for the switch from SSA2 to SSA1-SG1 in whitefly populations driving cassava virus pandemics in East and Central Africa. © 2013 Elsevier B.V.


Masunga R.H.,Lake Zone Agricultural Research and Development Institute | Masunga R.H.,Ghent University | Uzokwe V.N.,International Institute Of Tropical Agriculture | Mlay P.D.,Lake Zone Agricultural Research and Development Institute | And 4 more authors.
Applied Soil Ecology | Year: 2016

Sustainable agriculture requires the careful optimization of the use of organic amendments to improve soil fertility while minimizing any harmful environmental effects. To understand the events that occur in soil after the addition of different organic amendments, we evaluated the nitrogen (N) mineralization dynamics in soil after adding organic amendments, and evaluated changes in the microbial population. The four organic amendments were fresh dairy cattle manure, fresh white clover, vegetable, fruit, and yard waste compost, and poplar tree compost. The N mineralization potential of each organic amendment was determined by analyzing total mineral nitrogen during a 97-day laboratory incubation experiment. Soils amended with clover released 240 μg N g-1 soil during the 97-day incubation, more than twice as much as that released from soils amended with manure or composts (76-100 μg N g-1 soil). At the end of the incubation, the net N mineralization in clover-amended soils was 54%, more than five times higher than that in soils amended with composts or manure (4%-9%). Nitrogen was mineralized faster in clover-amended soil (1.056 μg N g-1 soil day-1) than in soil amended with composts (0.361-0.417 μg N g-1 soil day-1). The microbial biomass carbon content was higher in clover-amended soil than in the soils amended with manure or composts. We monitored changes in the microbial population in amended soils by a phospholipid fatty acid (PLFA) analysis. On day 97, there were higher concentrations of total PLFAs in soils with organic amendments (e.g., 14.41 nmol g-1 in clover-amended soil) than in control soil without amendments (9.84 nmol g-1). Bacteria (Gram-positive and Gram-negative), actinomycetes, and fungi were more abundant in clover-amended soils than soils amended with manure or composts. The N mineralization potential varied among the four organic amendments. Therefore, the timing of application and the type of organic amendment should be matched to the nutrient needs of the crop. © 2016 Z.


Hillocks R.,University of Greenwich | Maruthi M.,University of Greenwich | Kulembeka H.,Lake Zone Agricultural Research and Development Institute | Jeremiah S.,Lake Zone Agricultural Research and Development Institute | And 10 more authors.
Journal of Phytopathology | Year: 2016

Cassava brown streak disease is endemic to the coastal regions of East Africa, and from around 2004, the disease resurged and became epidemic in the Great Lakes Region, where it continues to spread. In both these areas, cassava brown streak disease (CBSD) leaf symptoms occur at high incidences. However, it is the associated symptom of root rot (necrosis) in the starch-bearing tissues that renders the root unfit for human consumption. Because the extent of root necrosis is not known until the crop is harvested and surveys require destructive sampling, root symptoms are much less frequently assessed than are the above-ground symptoms on the leaves and stems. Surveys were undertaken in selected villages in Tanzania, Kenya, Uganda and Malawi to assess the incidence of CBSD leaf symptoms and the incidence and severity of root symptoms, to estimate the impact of the disease on household food security and on cassava processing. CBSD leaf symptoms were recorded at high incidences (40-90% in individual fields) in all fields visited throughout East Africa, but root necrosis incidence was lower than would be expected from the high incidence of leaf symptoms. Severe root necrosis at high incidence was found only on a few varieties, usually grown to a limited extent. It appears that varieties that are prone to root necrosis are being abandoned in favour of those with a lower propensity to develop root necrosis after infection by the virus. © 2016 Blackwell Verlag GmbH.


Okello J.J.,International Potato Center | Sindi K.,International Potato Center | Shikuku K.,International Potato Center | Low J.,International Potato Center | And 5 more authors.
Agroecology and Sustainable Food Systems | Year: 2015

Efforts to combat vitamin A deficiency in children and pregnant mothers have focused on promotion of nutritionally enhanced food security crops. Such efforts have recently targeted the production and consumption of these crops. The orange-fleshed sweetpotato (OFSP) is one of the most known nutritionally enhanced crops. However, access to quality planting materials of OFSP and most vegetatively propagated crops by farmers remains a major constraint. This study used discrete and count data regression models and data collected from 732 farm households in Tanzania to test the effect of participation on a project designed to break this quality “seed” bottleneck (via technology awareness and targeted access to clean planting materials) on a) the decision to conserve planting materials during dry periods for future planting and b) the number of conservation strategies used by farmers. The study found that participation in such a project increases both the likelihood of conserving quality planting materials and also the number of conservation strategies employed by the farmers. It also found that varietal attributes and the agroecology of the area affect conservation of OFSP planting materials. It concludes that awareness and access to clean sweetpotato planting materials of nutritionally enhanced crops promote farmers. conservation of own planting materials. The study discusses the policy implications of the findings. Copyright © Taylor & Francis Group, LLC.

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